Simple as that, you can not install a package with higher EAPI then your portage supports.

yes, I need syntax where ever I can adjust eapi for necessary level.

It's easy: just create a local overlay, then rewrite every ebuild you want to install via that obsolete portage version you insist on using. Should be no problem for someone with such special requirements; here's TFM to get you started.

As has been said already, this is NOT a configuration or bash issue. Your installed version of sys-apps/portage simply cannot work with ebuilds using EAPI-5. Solution: Install a version of sys-apps/portage that does support EAPI-5. Modifying random files will only cause new problems as you have seen.